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Combustion engines which compress the combustion air have an ambient temperature sensitivityxe2x80x94both the capacity and the efficiency decrease as the ambient temperature increases. The power demand of the compressor section of the engine is approximately proportional to the absolute temperature of the inlet air, which makes the efficiency of the engine proportional to the inverse of the absolute temperature. The compressor capacity, and hence overall engine capacity, is proportional to the density of the inlet air.
The net result is that for a compressed air combustion engine, both the power output and engine efficiency are de-rated at warm ambients. The degradation is not so severe with reciprocating engines, which require little more than stoichiometric air. The degradation is very severe with combustion turbines, which require on the order of 3 or 4 times stoichiometric air.
One known method of counteracting the warm ambient degradation of compressed air combustion engines is by cooling the inlet air, either evaporatively or with a refrigerant. The refrigerated cooling can be done either in refrigerated air coils or by direct contact with sprayed chilled water. The refrigeration is supplied by either mechanical or absorption refrigeration systems, and in some instances through a cold storage medium (ice or chilled water).
Another approach to cooling combustion engine inlet air is by over-spraying, typically via fogging. Sufficient water is injected into the air in fine droplet form such that it not only reduces the temperature adiabatically to the dew point, but additional droplets remain un-evaporated, and carry into the engine compressor. Those droplets rapidly evaporate as compression proceeds, slowing the temperature increase caused by compression, and hence effectively adding to the amount of inlet cooling. For the droplets to remain suspended in the air into the compressor rather than separate out excessively, they should be in the fog-size range, i.e., less than 40 microns in diameter and preferably 5 to 20 microns. Another advantage of this size range is that the droplets are small enough that they do not erode the compressor blades.
The problems with the current approaches to cooling engine compressor inlet air include the following. Most compressors would benefit thermodynamically from sub-freezing inlet temperatures, or at least could be designed to benefit from those temperatures. However, there are many practical difficulties. Especially with high rotational speed combustion turbines, there is a possibility of ice buildup on inlet guide vanes, which then could spall off and damage the compressor blades. This imposes a practical limiting temperature of about 4xc2x0 C. for many inlet cooling systems. Cooling below that temperature will require some additional technique of reducing the humidity level of the cold air below saturationxe2x80x94reheat, etc. On the refrigeration side, special measures are also required to deal with the H2O removal from the air in sub-freezing conditions: periodic defrosting of the air coils, or continuous addition of a melting agent. Furthermore, the refrigeration system requires proportionately more input power to reach the lower temperaturesxe2x80x94more shaft power for mechanical refrigeration, or higher quality heat for absorption refrigeration. With mechanical refrigeration, the power necessary to reach sub-freezing temperatures is so large, and the marginal improvement in the engine due to colder compression is so small, that there is little or no net gain from cooling to sub-freezing temperatures.
Even when the inlet cooling is restricted to above-freezing temperatures, other major problems remain. The compressor benefit is substantially due to the sensible cooling of the inlet air, with almost no added benefit from the latent cooling, i.e., the amount of moisture condensed out of the air. However, the latent cooling typically represents 25 to 50% of the total refrigeration load. For example, consider 35xc2x0 C. air at 50% relative humidity, which is cooled to 5xc2x0 C. at 100% relative humidity. The moisture content decreases from 1.8 weight percent to 0.55 weight percent. For these conditions, only 51% of the total refrigeration provides sensible cooling, and 49% causes the water condensation. Thus, much of the refrigeration is effectively wasted.
Another problem is that the water removal results in reduced mass flow through the turbine, proportionately reducing its power output. Air flow can be correspondingly increased, but that adds compression power.
The overspray or fogging approach to inlet cooling also presents problems. The two foremost are that the cooling is adiabatic, as opposed to the diabatic cooling of the refrigeration approach; and that a source of pure water is required for every bit of cooling accomplished. The adiabatic limitation causes the inlet sensible temperature to be no lower than the dew point. The cost and availability of pure water mitigate against this approach at many sites.
It is known that injection of some water, as either vapor or liquid, into the compressed air of a combustion turbine increases the capacity and decreases the emissions and heat rate. However, a costly supply of pure water is required.
What is needed, and included among the objects of this invention, are apparatus and process which overcome the prior art problems cited above, i.e., an inlet cooling system wherein the latent load contributes to power augmentation and heat rate improvement in addition to the sensible load contribution; where the benefits of the water injection are available without the limitations of needing a large source of pure water and that the inlet temperature is limited to the dew point; where the thermodynamic benefits of sub-freezing inlet temperatures are achievable without the practical problems; and wherein the refrigeration system is activated by low temperature waste heat so as not to detract from the compressor shaft power reduction (system power gain) provided by the inlet cooling system.
The Nagib ""71 article shows that recuperated combustion turbines derive the maximum benefit from inlet cooling. Recuperation causes lower exhaust temperatures, and inlet cooling causes a further reduction in exhaust temperature. Similarly, cogeneration and combined cycle configurations have very low exhaust temperatures. Prior art waste heat-activated absorption inlet cooling cycles require exhaust temperatures of about 200xc2x0 C. or higher. For the more aggressive spray cooling disclosed here, such temperatures will not usually be available. Thus, one important aspect of this disclosure is the identification of an absorption cycle which can be powered by waste heat well below 200xc2x0 C.
In order to condense moisture out of the exhaust, it must be cooled to well below 80xc2x0 C. It would be advantageous if the absorption cycle heat input caused that low a temperature, to minimize any need for additional ambient cooling of the exhaust.
This disclosure recites a compressed air combustion engine with inlet cooling supplied by an absorption unit powered by the combustion exhaust. Moisture is condensed from the inlet air and/or from the exhaust, and is sprayed into the compressed air and/or fogged into the chilled inlet air. The special flow sequence desirable in a two-pressure absorption cycle in this service is disclosed, as well as the three-pressure absorption cycle which provides maximum thermodynamic benefit.
In particular, an apparatus for energy conversion is disclosed comprised of:
a) a combustion engine comprised of a compressor, a combustor, and a work expander;
b) an engine inlet air chiller;
c) an absorption refrigeration unit which supplies chilling medium to said chiller;
d) an exhaust heat exchanger which transfers heat to said ARU from said engine exhaust;
e) a means for collecting condensate from at least one of said chiller and said exhaust heat exchanger; and
f) a means of injecting at least part of said collected condensate into said compressor discharge vapor.
In another embodiment, the energy conversion apparatus is comprised of:
a) a combustion engine with a combustion air compressor;
b) an absorption refrigeration unit (ARU) which is powered by engine exhaust;
c) a sequential path for absorbing solution in said ARU comprised of the following components in sequence:
i) a solution pump;
ii) a solution cooled vapor rectifier (SCVR);
iii) a solution heat exchanger (SHX);
iv) an exhaust heated once-through co-current vapor generator; and
v) a vapor liquid separator which sends vapor to said SCVR and liquid to said SHX.